METHOD FOR THE PRODUCTION OF POTASSIUM CHLORIDE GRANULES

Description

[0001] The present invention relates to a process for producing potassium chloride granules from a crystalline potassium chloride raw material, for example from crystalline potassium chloride obtained by flotation, evaporation, crystallization, solar evaporation, or a hot dissolution process. The invention also relates to the potassium chloride granules obtainable by the process.

[0002] Potassium chloride is an important component of agricultural fertilizers. Potassium chloride is typically extracted from underground mines through conventional mining, solution mining, or solar evaporation of salt water. The resulting potassium chloride is then further processed into the desired application forms.

[0003] Potassium chloride is often marketed in granular form due to its advantageous handling properties. Compared to fine-particle crystalline potassium chloride, granules are much less prone to dust formation, are more stable in storage, less prone to caking, and, when used as fertilizer, are easier and more evenly spread by spreading. The quality of the potassium chloride granules and thus the achievable price on the market depend on both the purity and the quality of the granules.

[0004] The crystalline potassium chloride raw material resulting from the mining of potassium chloride typically has particle sizes significantly below the desired granule size. To produce the granules, the potassium chloride raw materials are subjected to a conventional granulation process in which the finely divided crystalline potassium chloride particles of the raw material are agglomerated, resulting in increased grain size.

[0005] Common granulation processes for the production of potassium chloride granules are press agglomeration and build-up agglomeration. In build-up agglomeration of potassium chloride, the finely divided starting material is vigorously agitated with the addition of an aqueous liquid, resulting in numerous collisions between the primary particles, which then aggregate due to the capillary forces mediated by the liquid. These aggregates can then aggregate with each other or with other primary particles. The constant agglomeration leads to a continuous build-up of particle layers and the compaction of the particles, ultimately resulting in moist granules (green granules) of the desired size, which are then dried and cured to form the finished granules.During the press agglomeration of potassium chloride, the finely divided starting material is compacted by applying pressure, which sometimes exerts very high forces on the primary particles. This leads to deformation of the primary particles in the contact area, for example, through plastic deformation, which significantly increases the adhesion of the primary particles to one another. Frictional heat can also cause local sintering, resulting in the formation of solid bridges between the primary particles.

[0006] The actual agglomeration is followed, if necessary after drying of the moist granulate, by a classification of the primary granulate obtained, in which the primary granulate is separated into fractions of the desired particle size.

[0007] Potassium chloride granules are generally mechanically unstable. When exposed to mechanical forces, such as those encountered during handling, storage, or especially during transport, the granule particles become damaged. This leads, on the one hand, to a reduction in the particle diameter of the granule particles and a consequent loss of value, and, on the other hand, to a significant formation of fine particles. These fine particles can cause problems during storage and handling of the granules, for example, by generating dust or, in the presence of moisture, causing the granule particles to caking together.

[0008] To improve the mechanical stability of the granules, binders are often used in the aforementioned granulation processes. These binders improve the adhesive forces between the particles and thus the cohesion of the particles in the granules. Typical binders are gelatin, starch, molasses, lignin sulfonates, lime, and clay minerals. The choice of binder will generally significantly influence the properties of the granules, particularly their mechanical strength (abrasion, hardness), their hygroscopic properties, and their tendency to generate dust. However, even when using such conventional binders, potassium chloride granules usually exhibit insufficient mechanical stability, leading to the problems mentioned above.

[0009] SU 990755 describes a process for producing potassium chloride granules by a press agglomeration process, in which sodium polyphosphate is added to the potassium chloride starting material in an amount of 0.2 to 1 wt.% based on potassium chloride.

[0010] RU 2083536 describes a process for the production of potassium chloride granules by press agglomeration of potassium chloride raw material, in which the potassium chloride dust resulting from the press agglomeration is mixed with an aqueous solution of sodium metasilicate and added to the potassium chloride raw material used for compaction.

[0011] US 4,385,020 describes a process for producing potassium chloride granules in which potassium chloride is processed with a phosphate binder in a drum or disc granulator to form potassium chloride granules.

[0012] DE 10252848 describes a process for producing potash fertilizer granules, in which raw potash fertilizer granules are treated with a silicate- or carbonate-containing solution. The treated granules are then subjected to kinetic energy in the form of vibrations. The resulting granules are coated with a water-repellent substance, such as palmitic acid, to improve their resistance to atmospheric moisture.

[0013] CA 2,465,461 describes a process for producing potassium chloride granules using a press-agglomeration process in which hexasodium metaphosphate (SHMP), tetrasodium pyrophosphate, or trisodium phosphate is added to the potassium chloride as a binder prior to compaction. The SHMP is intended to bind the moisture and the magnesium and calcium salts contained in the potassium chloride, thus improving its mechanical strength, particularly during transport.

[0014] US 2005 / 036929 A1 describes a process for producing potassium chloride granules by compacting a potassium chloride raw material to which sodium hexametaphosphate has previously been added as a binder.

[0015] However, despite these binders, the mechanical properties of the potassium chloride granules known from the prior art are unsatisfactory, especially when the potassium chloride granules are exposed to an atmosphere with elevated humidity for an extended period. In particular, the known potassium chloride granules are characterized by insufficient fracture or bursting strength and unsatisfactory abrasion after storage at elevated humidity.

[0016] The present invention is therefore based on the object of providing potassium chloride granules with improved mechanical strength, in particular high breaking or bursting strength, and satisfactory abrasion values. In particular, the potassium chloride granules should still exhibit satisfactory or good mechanical properties even after prolonged exposure to high humidity, for example, humidity of 70% RH (relative humidity) or higher, or other moisture exposure as a result of weathering. This means that the breaking or bursting strength should be high even at high humidity, and moisture absorption should be low.

[0017] It has surprisingly been found that these objects can be achieved by treating a crystalline potassium chloride raw material in the presence of water, for example in the form of a filter-moist fine salt, with at least one alkali metal carbonate and at least one phosphate additive before granulation, the water content during the treatment being in the range from 2 to 15% by weight, based on the solid constituents of the potassium chloride raw material, and the alkali metal carbonate being used in an amount of 0.05 to 1% by weight and the phosphate additive in an amount of 0.025 to 2% by weight, based on the solid constituents of the potassium chloride raw material. By combining at least one alkali metal carbonate and at least one phosphate additive, selected from alkali metal monophosphates, alkali metal pyrophosphates, linear alkali metal polyphosphates and mixtures thereof, the fracture orBursting strength is significantly increased at higher air humidity levels of, for example, 70% RH or higher and the moisture absorption of the potassium chloride granules is reduced.

[0018] Accordingly, the present invention relates to a process for the production of potassium chloride granules from a crystalline potassium chloride raw material, in which the potassium chloride raw material is treated, before granulation, with at least one alkali metal carbonate and at least one phosphate additive selected from alkali metal monophosphates, alkali metal pyrophosphates, linear alkali metal polyphosphates and mixtures thereof, in the presence of water, wherein the water content during the treatment of the crystalline potassium chloride raw material with the alkali metal carbonate and the phosphate additive is in the range of 2 to 15 wt.%, based on the solid components of the potassium chloride raw material, and wherein the alkali metal carbonate is used in an amount of 0.05 to 1 wt.% and the phosphate additive is used in an amount of 0.025 to 2 wt.%, based on the solid components of the potassium chloride raw material.

[0019] Preferred embodiments of the method according to the invention are described in the subclaims and below.

[0020] The potassium chloride granules obtainable according to the invention are characterized by higher mechanical strength, in particular higher breaking or bursting strength, compared to potassium chloride granules made from untreated crystalline potassium chloride raw material and also compared to potassium chloride granules made from potassium chloride raw material that was only treated with an additive, i.e. with either the alkali metal carbonate or the phosphate additive, before granulation. The granules are also characterized by low abrasion. The advantageous mechanical strength is particularly evident when the potassium chloride granules are exposed to moisture as a result of weathering, e.g. an atmosphere with increased air humidity, in particular an air humidity of 70% RH or higher. This is particularly surprising because treatment with alkali metal carbonate alone does not lead to any significant improvement in breaking or bursting strength.Burst strength values of weathered granules.

[0021] Accordingly, the present invention also relates to the potassium chloride granules obtainable by the process according to the invention.

[0022] The invention also relates to the use of a combination of at least one alkali metal carbonate, at least one phosphate additive and water for increasing the breaking / bursting strength and reducing the moisture absorption of potassium chloride granules.

[0023] The present invention further relates to the use of a combination of at least one alkali metal carbonate, at least one phosphate additive and water for increasing the breaking or bursting strength of potassium chloride granules exposed to an air humidity of or greater than 70% RH.

[0024] In the process according to the invention, a crystalline potassium chloride raw material is used as the starting material. This crystalline potassium chloride raw material is also referred to below as fine salt. The crystalline potassium chloride raw material consists essentially of potassium chloride, i.e. generally to at least 90% by weight, frequently to at least 95% by weight, in particular to at least 98% by weight and especially to at least 99% by weight or at least 99.5% by weight, based on the solid constituents of the crystalline potassium chloride raw material. The potassium content of the potassium chloride raw material, calculated as K 2 O, is usually at least 56.9% by weight, frequently to at least 60.0% by weight, in particular to at least 61.9% by weight and especially to at least 62.5% by weight, based on the solid constituents of the potassium chloride raw material.

[0025] Depending on its origin, the potassium chloride raw material contains the typical impurities, in particular sodium salts and alkaline earth metal salts, especially magnesium salts and / or calcium salts. It can be assumed that these impurities, in particular the magnesium and calcium salts, lead to the observed stability problems of the granules, especially when the granules are exposed to high humidity. The potassium chloride raw material used often contains alkaline earth metal salts, e.g. calcium and / or magnesium salts, in a total amount of 0.01 to 1.0 wt.%, in particular 0.05 to 0.7 wt.%, each calculated as alkaline earth metal chloride, e.g. as MgCl 2 or CaCl 2 , and based on the potassium chloride (KCl) contained in the raw material.

[0026] The potassium chloride raw material used to produce the granules is typically crystalline potassium chloride mined or obtained via solar evaporation or solution mining, which has been processed, for example, by flotation, evaporation, crystallization and / or a hot dissolution process, or by a combination of these measures. In the process according to the invention, additional potassium chloride can also be admixed with the potassium chloride raw material. This can be, for example, a waste material accumulating during the classification of the potassium chloride granules according to the invention, which may have been comminuted. In these mixtures of potassium chloride raw material and additional potassium chloride, the proportion of additional potassium chloride, e.g. the waste material, will generally be in the range of 1 to 70 wt. %, based on the total mass of the quantity added for granulation.

[0027] Instead of a freshly processed fine salt / potassium chloride raw material, a ready-made fine salt can also be used for granulation, for example a ready-made fine salt with a potassium content of at least 60 wt.%, based on the dry matter and calculated as K2O.

[0028] Typically, the potassium chloride raw material is in the form of fine crystalline salt particles. In addition to the crystal particles, the potassium chloride raw material may also contain coarser particles, e.g., from the return material. Typically, a potassium chloride raw material is used in which at least 90% by weight of the potassium chloride raw material particles have a maximum particle size of 2 mm. In particular, 90% by weight of the potassium chloride raw material particles have a particle size in the range of 0.01 to 2 mm.

[0029] According to the invention, the potassium chloride raw material is treated with at least one phosphate additive and at least one alkali metal carbonate in the presence of water prior to granulation. The alkali metal carbonate and phosphate additive are also referred to below as additives.

[0030] The treatment of the potassium chloride raw material with alkali metal carbonate and phosphate additives can be carried out simultaneously or successively. If added simultaneously, the alkali metal carbonate and phosphate additives can be added separately or as a premix.

[0031] Examples of suitable alkali metal carbonates are sodium carbonate and potassium carbonate, which can be used in anhydrous form or in the form of their hydrates. In particular, the alkali metal carbonate is selected from anhydrous sodium carbonate (Na 2 CO 3 ), sodium carbonate monohydrate (Na 2 CO 3 * H 2 O), and sodium carbonate decahydrate (Na 2 CO 3 * 10 H 2 O), and mixtures thereof. The particularly preferred alkali metal carbonate is anhydrous sodium carbonate.

[0032] Suitable phosphate additives are those of the formula M n+2 P n O 3n+1 , where M is an alkali metal cation, e.g. Na or K, and n is 1 - 100, in particular 1, 2 or 3, namely alkali metal monophosphates (n = 1), alkali metal pyrophosphates (n = 2) and linear alkali metal polyphosphates (n > 2). Preference is given to the corresponding sodium compounds, ie M stands for sodium. Examples of preferred phosphate additives are, in particular, trisodium phosphate, trisodium phosphate dodecahydrate, trisodium phosphate hexahydrate, sodium pyrophosphate (anhydrous), sodium pyrophosphate dodecahydrate, sodium tripolyphosphate (STPP) and mixtures thereof. A particularly preferred phosphate additive is trisodium phosphate. Instead of the aforementioned sodium compounds, it is also possible to use phosphate additives in which sodium ions are completely or partially replaced by potassium.

[0033] In the process according to the invention, the alkali metal carbonate is used in an amount of 0.05 to 1 wt.%, and especially in an amount of 0.1 to 0.7 wt.%, based on the solid components of the potassium chloride raw material. In particular, the amount of alkali metal carbonate used depends on the alkaline earth metal salts contained in the potassium chloride raw material.

[0034] In the process according to the invention, the phosphate additive is used in an amount of 0.025 to 2 wt.%, in particular in an amount of 0.05 to 1.5 wt.%, and especially in an amount of 0.07 to 0.4 wt.%, based on the solid constituents of the potassium chloride raw material. In particular, the amount of phosphate additive used depends on the content of alkaline earth metal salts present in the potassium chloride raw material.

[0035] It is essential that the treatment of the potassium chloride raw material with the alkali metal carbonate and the phosphate additive takes place in the presence of water. This can be water that originates from the processing of the potassium chloride raw material, for example water adhering to or enclosed in the potassium chloride particles, or the water of crystallization, and / or water that was added to the potassium chloride raw material before or during the addition of the alkali metal carbonate or phosphate additive. The total water content in the potassium chloride raw material during treatment with the alkali metal carbonate and the phosphate additive is in the range of 2 to 15 wt.%, in particular in the range of 4 to 9 wt.%, in each case based on the solid constituents of the potassium chloride raw material. If the total water content in the potassium chloride raw material before treatment with the alkali metal carbonate and the phosphate additive is less than 2 wt.-%, based on the solid components of the potassium chloride raw material, it will be increased, for example, before or during the treatment by adding water to a value of at least 2 wt.%, in each case based on the solid components of the potassium chloride raw material.

[0036] A common approach is to use a moist potassium chloride raw material that already has the desired water content. If necessary, the water content of the potassium chloride raw material is adjusted to these values before or during treatment with the alkali metal carbonate and the phosphate additive.

[0037] During the treatment of the potassium chloride raw material, the at least one alkali metal carbonate and the at least one phosphate additive can be added to the potassium chloride raw material simultaneously or successively. It is fundamentally irrelevant whether the alkali metal carbonate is added first and then the phosphate additive to the potassium chloride raw material, or vice versa, or whether the alkali metal carbonate and phosphate additive are added simultaneously to the potassium chloride raw material. It is essential that the alkali metal carbonate and phosphate additive are added before granulation and in the presence of a sufficient amount of water. If the potassium chloride raw material is dried before granulation, the alkali metal carbonate and phosphate additive, and optionally water, are typically added to the potassium chloride raw material before drying.

[0038] A common procedure involves adding the alkali metal carbonate additive and the phosphate additive to the moist potassium chloride raw material, and then drying the thus-treated moist potassium chloride raw material, i.e., treated moist fine salt, prior to granulation, particularly when granulation is carried out by compression granulation. In particular, drying is carried out to a maximum water content of 1 wt.%, based on the solid components in the thus-treated potassium chloride raw material. Granulation is then carried out. The thus-treated and dried potassium chloride raw material can also be stored prior to granulation.

[0039] To treat the potassium chloride raw material with at least one alkali metal carbonate, the alkali metal carbonate is generally used in the form of a powder and / or an aqueous solution. If the alkali metal carbonate is used in powder form, the particle size of the powder will generally not exceed 1 mm and in particular 0.5 mm. If the total water content of the potassium chloride raw material is insufficient, the addition of alkali metal carbonate as a solution is also possible.

[0040] To treat the potassium chloride raw material with at least one phosphate additive, the phosphate additive is generally used in the form of a powder and / or an aqueous solution. If the phosphate additive is used in powder form, the particle size of the powder will generally not exceed 1 mm and in particular 0.5 mm. If the total water content of the potassium chloride raw material is insufficient, the addition of the phosphate additive as a solution is also possible.

[0041] To treat the potassium chloride raw material with the alkali metal carbonate and the phosphate additive, the alkali metal carbonate or the phosphate additive is usually mixed with the potassium chloride raw material in the desired amount. As already mentioned, this mixing must take place before granulation. The total water content in the moist potassium chloride raw material during the addition of alkali metal carbonate and phosphate additive should be in the range of 2 to 15 wt.% and especially in the range of 4 to 9 wt.%, in each case based on the solid components of the potassium chloride raw material, or should be adjusted to these values. In particular, the alkali metal carbonate is added to the moist potassium chloride raw material (i.e., the moist fine salt) before drying. In a special embodiment, both the alkali metal carbonate and the phosphate additive are added to the moist potassium chloride raw material (i.e.,Add the desired amount to the moist fine salt before drying.

[0042] Furthermore, the process according to the invention can also be used to produce potassium chloride granules that additionally contain micronutrients such as B, Mn, Mo, Cu, Zn, and Fe, or mixtures thereof. The micronutrients can be added before, during, or after granulation. For example, a potassium chloride raw material that already contains the desired amount of micronutrients can be used. Frequently, however, the micronutrients are added during the process according to the invention, e.g., during or after the addition of the additives, and the resulting potassium chloride raw material is then granulated. The micronutrients can also be added to the finished granules, for example, by spraying an aqueous solution of the micronutrients onto the granules. The amount of micronutrients will generally not exceed 1% by weight, based on the anhydrous potassium chloride granules and calculated as an element.For example, the potassium chloride granules obtainable according to the invention may contain 0.001 to 1 wt.% B.

[0043] The actual granulation can be carried out analogously to the agglomeration processes known from the state of the art, which are described, for example, in Wolfgang Pietsch, Agglomeration Processes, Wiley - VCH, 1st edition, 2002 as well as in G. Heinze, Handbook of Agglomerations Technology, Wiley - VCH, 2000 and in Perry's Chemical Engineers' Handbook, 7th edition, McGraw-Hill, 1997.

[0044] Granulation is usually carried out as a pressing or build-up agglomeration.

[0045] In granulation by means of built-up agglomeration, the treated potassium chloride raw material, which contains the alkali metal carbonate and phosphate additives in the desired amounts, is set in motion by the action of mechanical forces and optionally treated with water or aqueous solutions of alkali metal carbonate and phosphate additive during the granulation process. The built-up agglomeration can be carried out in a conventional manner as a rolling, mixed, or fluidized bed agglomeration, in particular as a rolling agglomeration. In rolling agglomeration, the potassium chloride raw material, which may already contain the alkali metal carbonate and phosphate additive components, is placed in a vessel with an inclined axis of rotation and a circular cross-section, preferably in a granulation drum or on a granulation plate. The particles of the fine salt are set in motion by rotating the vessel. The treatment with the water orThe aqueous solutions of alkali metal carbonate and phosphate additive are applied, for example, by spraying them onto the agitated potassium chloride raw material. This produces a relatively uniformly round granulate that can be directly fed to a classification system.

[0046] Preferably, the granulation process comprises press agglomeration of the treated potassium chloride raw material and comminution of the material obtained during the press agglomeration. During the press agglomeration, the treated potassium chloride raw material is compacted using pressure. In principle, all presses known for similar purposes are suitable for compaction, such as ram presses, extruders, punch presses, and roller presses.

[0047] Compaction is preferably carried out using a roller press. In roller presses, compaction occurs in the gap between two counter-rotating rollers. The roller surfaces can be smooth, profiled (e.g., corrugated, wavy, or waffled), or equipped with molded cavities. Any profiling of the roller surface primarily serves to improve the feed ratio into the roller gap. Roller presses with smooth or profiled roller surfaces are often used. In this case, the primary agglomeration product is a ribbon-like strand emerging from the roller gap, also known as a slug.

[0048] The pressing forces required for compaction, which are usually related to the roller width and specified as line forces, are generally in the range from 1 to 75 kN / cm, in particular in the range from 40 to 70 kN / cm and related to a 1000 mm diameter and an average flake thickness of 10 mm. The roller press is generally operated at a roller circumferential speed in the range from 0.2 to 1.6 m / s. Compaction usually takes place at temperatures in the range from 80 to 100 °C or at the temperature that is established due to the effect of the mechanical forces on the treated potassium chloride raw material (i.e. the treated fine salt). If necessary, the material fed to the granulation is preheated to the temperature required for compaction or still has residual heat, e.g. from drying.

[0049] If necessary, the press agglomeration can be carried out in several stages.

[0050] During the press agglomeration of the treated potassium chloride raw material using a roller press, flakes are usually obtained, which are then subjected to comminution to adjust the particle size of the resulting granulate. The comminution of the flakes can be carried out in a conventional manner, for example, by grinding in suitable devices, such as impact crushers, impact mills, or roller crushers.

[0051] Typically, the actual granulation process is followed by classification of the granules. This involves separating the granules into granules with the specified particle size, smaller granules (fine fraction), and, if necessary, coarser granules (coarse fraction). Potassium chloride granules are considered to be compliant with the specifications if at least 90% by weight of the granule particles have a particle size or diameter in the range of 0.5 to 8 mm, and especially in the range of 2 to 4 mm. Classification can be carried out using conventional methods, in particular by sieving.

[0052] The granulate material that does not meet the specifications and is generated during classification, the so-called return material, is usually returned to the process.

[0053] In a preferred embodiment of the invention, a moist potassium chloride raw material containing 2 to 15 wt. %, in particular 4 to 9 wt. % water, based on the solid constituents of the potassium chloride raw material, is mixed with the at least one alkali metal carbonate, in particular anhydrous sodium carbonate, and the at least one phosphate additive, in particular trisodium phosphate, in the desired amount, to obtain a treated (conditioned) moist potassium chloride raw material. Alkali metal carbonate and trisodium phosphate can be used in the form of solids or in the form of aqueous solutions. The conditioned potassium chloride raw material thus obtained is subsequently dried. The dry, conditioned potassium chloride raw material is optionally fed with the return material to a press agglomeration, in particular to a press agglomeration using a roller press with smooth or profiled rollers.The resulting granulate, or flakes, are then crushed and classified. The fines resulting from the classification are fed to the press agglomeration together with the dried, conditioned potassium chloride raw material.

[0054] The granulate thus obtained can be processed, e.g., packaged and transported, in a conventional manner.

[0055] The potassium chloride granules obtainable by the process according to the invention naturally contain, in addition to potassium chloride, the alkali metal carbonate additive and the phosphate additive (or their conversion products) in the amount used in the process according to the invention. In particular, the potassium chloride granules obtainable by the invention consist of at least 90% by weight, in particular at least 95% by weight, and especially at least 98% by weight, based on anhydrous granules: i) potassium chloride, ii) the additive alkali metal carbonate and / or its reaction products such as MgCO 3 or CaCO 3 , in an amount of 0.05 to 1 wt.%, in particular in an amount of 0.1 to 0.7 wt.%, based on the potassium chloride contained in the granulate and calculated as alkali metal carbonate, and iii) the phosphate additive or its hydrolysis / conversion products, in an amount of 0.025 to 2 wt.%, in particular in an amount of 0.05 to 1.5 wt.%, based on potassium chloride and calculated as phosphate additive.

[0056] In addition, the potassium chloride granulate obtainable according to the invention contains the impurities contained in the potassium chloride raw material / fine salt, e.g., magnesium salts and / or calcium salts in the proportions specified above.

[0057] Furthermore, the potassium chloride granules may also contain micronutrients such as B, Mn, Mo, Cu, Zn, and Fe, or mixtures thereof. The amount of micronutrients will generally not exceed 1 wt.%, based on the anhydrous potassium chloride granules and calculated as an element. For example, the potassium chloride granules obtainable according to the invention may contain 0.001 to 1 wt.% boron.

[0058] As already mentioned, the potassium chloride granules according to the invention are characterized by high mechanical stability even when stored in a humid atmosphere, e.g., at relative humidity / humidity levels of 70% RH or above 70% RH. Even under these conditions, the potassium chloride granules according to the invention exhibit only low dust behavior and high fracture / burst strength, low moisture absorption, and low abrasion.

[0059] Figure 1shows a test arrangement for determining the "breaking strength" for test specimens comprising a test punch (1) with a conical test tip (R5) and a U-shaped test specimen holder (3) in which the test specimen (2) is fixed on both sides. Laboratory tests:

[0060] A crystallizate obtained by hot dissolving was used as the potassium chloride raw material (fine salt). The potassium content of the potassium chloride was approximately 60 wt.%, calculated as K2O and based on solid components. The Mg content, calculated as MgCl2, and the Ca content, calculated as CaCl2, totaled approximately 0.13 wt.%, based on solid components. The grain size of the potassium chloride raw material (fine salt) was generally between 0.01 and 2 mm. The water content of the moist potassium chloride raw material (moist fine salt) was 4–9 wt.%, in particular 8 wt.%, based on the solid components before drying.

[0061] As alkali metal carbonate and phosphate additive, a commercially available powdered sodium carbonate anhydrous, trisodium phosphate (without water of crystallization) and sodium pyrophosphate (without water of crystallization) with a water content of 0.01 wt.% were used, respectively.

[0062] Preparation of test specimens for determining fracture strength: 3 kg of the potassium chloride powder specified above was mixed with the respective additive in an intensive mixer for 1 minute with the addition of 240 g of water. The moist potassium chloride raw material / additive mixture was dried for 24 hours in a drying cabinet at 105 °C and then deagglomerated to a grain size of < 0.8 mm using a disc mill. For the "dry" comparative tests, the additives were mixed in after drying and after deagglomeration.

[0063] To determine the breaking strength, cuboidal test specimens measuring 50 x 50 x 8 mm were produced from this material. The test specimens (laboratory tests) were produced using a hydraulic stamping press (model K50 from Komage) with a pressing force of approximately 290 kN, as described in Figure 1 shown schematically.

[0064] Determination of the breaking strength (point load) of the test specimens: The unweathered test specimens were measured immediately after their production.

[0065] For weathering, the freshly prepared test specimens were weighed and then weathered as follows: The test specimens were fixed vertically in sample holders and stored in a climate cabinet for 72 h at 20 °C and 70 % relative humidity.

[0066] Immediately after removal from the climate chamber, the weathered test specimens were weighed again to determine the water / moisture absorption and then the breaking strength was immediately determined.

[0067] The determination of the breaking strength via a point load was carried out in accordance with ASTM D5731:2008 (Point load strength index). For this purpose, the square test specimens (2) were placed in the U-shaped specimen holder (3) of the Figure 1The test device shown schematically was fixed on both sides so that the test tip (R5) was aligned with the center of the square test specimen (2). The test tip was then pressed onto the test specimen at a speed of 1 mm / min, and the force exerted on the test specimen was determined using a pressure gauge. The value determined was the maximum load on the test specimen immediately before fracture of the test specimen, which is characterized by a drop in force towards zero. The test tip was conical with a cone angle of 60°. The tip had a radius of 5 mm (cf. Fig. 1 ).

[0068] Ten test specimens (weathered and unweathered) were measured. The values for the ultimate strength (point load) given in Table 1 are averages of ten measurements. Tab.1: Fracture strengths of test specimens made of potassium chloride raw material and the additives sodium carbonate anhydrous and trisodium phosphate (anhydrous) or pyrophosphate (anhydrous), laboratory tests (square test specimens): # Additive Point loads - unweathered Point load ventilated** Moisture absorption at 70% RH ** 1* 0.16 wt% A11 + 0.32 wt% P941 0.40 kN 0.32 kN 0,18 % 2* 0.16 wt% A11 + 0.16 wt% P941 0.43 kN 0.31 kN 0,09 % 3* 0.16 wt% A11 + 0.08 wt% P941 0.36 kN 0.26 kN 0,14 % V4* 0.32 wt% P941 0.38 kN 0.30 kN 0,22 % V5* 0.16 wt% P941 0.39 kN 0.25 kN 0,34 % V6* 0.08 wt% P941 0.38 kN 0.22 kN 0,50 % V7* 0.04 wt% P941 0.36 kN 0.20 kN 0,71 % V8 0.32 wt% P941 dry 0.38 kN 0.22 kN 0,34 % V9* 0.13 wt.% A11 0.33 kN 0.19 kN 0,41 % V10 0.16 wt.% A11 dry 0.33 kN 0.15 kN 0,68 % V11 0.16 wt% A11 (dry) + 0.32 wt% P941 (dry) 0.37 kN 0.22 kN 0,22 % V12 Potassium chloride raw material (without additive) 0.34 kN 0.17 kN 0,63 % 13* 0.16 wt% A11 + 0.16 wt% P941 + 0.20 wt% P945 0.35 kN 0.26 kN 0,14 % *each with 8 wt.% water; ** Weathered for 72 h, 20 °C, 70% RH; # = test number; V = comparison test; A11 = sodium carbonate anhydrous; P941 = trisodium phosphate (anhydrous), P945 = sodium pyrophosphate (anhydrous) Factory operation trial:

[0069] To produce potassium chloride granules in a factory trial, moist potassium chloride raw material (i.e., moist fine salt) with a residual moisture content of 2–15 wt.% was fed to the drying process, possibly via a mixer. The additives according to the invention were added, for example, in the built-in mixer, and the mixture was homogenized. The treated fine salt was then fed to the drying process and subsequently, if necessary, fed to the presses with the press residue in the granulation process. After classification / crushing, the acceptable grain, the saleable potassium chloride granules, were obtained. This granule is known, among other things, as commercially available "60 MOP granules" if the potassium chloride content is at least 60.0% K2O.

[0070] For press agglomeration in production, several roller presses with a return material circulation system were used. The individual roller presses are designed as follows: two counter-rotating rollers have a waffle profile on the roller surface (typical roller diameter 1000 mm, typical working width 1000 mm, gap width typically approx. 15 mm). The press was operated with a line force of approximately 60 kN / cm and a roller speed of 18 rpm. The fine salt was generally fed via a central chain conveyor and the screw conveyors arranged above the presses.

[0071] The slugs produced in the roller press were crushed using an impact mill. The material was then classified using a commercially available screening device. The fraction with a grain size of 2-4 mm (product) was separated, the fraction with a grain size of < 2 mm was returned to the feed (fines), and the fraction with a grain size of > 4 mm (coarse material) was ground and screened again.

[0072] To determine the bursting strength of the granules, a test fraction (test granules) with a grain size of 2.5 - 3.15 mm was sieved.

[0073] The unweathered test granulate was measured parallel to the weathered granulate.

[0074] For weathering, approximately 9 g of the prepared test granules were placed in a Petri dish and weighed. For conditioning, the Petri dish was stored in a climate chamber for 24 hours at 20 °C and relative humidity levels of 70, 71, 72, or 73. Immediately after removal from the climate chamber, the Petri dish containing the test granules was reweighed to determine water absorption, and the breaking strength of the granules was then immediately determined using the following method.

[0075] The average bursting strengths were determined using the ERWEKA TBH 425D tablet bursting strength tester based on measurements of 56 individual agglomerates of varying particle sizes (fraction 2.5–3.15 mm), and the mean value was calculated. The force required to break the granules between the punch and the plate of the breaking strength tester was determined. Granule particles with a bursting strength of > 400 N and those with a bursting strength of < 4 N were not considered when calculating the mean value.

[0076] In the factory trial listed in Table 2, potassium chloride raw material with the following specifications was used: KCl content of approximately 61% K2O, approximately 0.2 wt.% MgCl2 / CaCl2 content, and the residual moisture content of the (moist) potassium chloride raw material is typically 5.7–6.2 wt.%. The processed quantities amount to approximately 90 t / h of potassium chloride raw material. Table 2: Factory trials of potassium chloride granules with anhydrous sodium carbonate and trisodium phosphate dodecahydrate from filter-moist potassium chloride (KCl) raw material* under different weather conditions (bursting strengths in N and moisture absorption in %) Additive unweathered 1 day / 70%RH 1 day / 71%RH 1 day / 72%RH 1 day / 73%RH 14* 0.16 wt% A11 + 0.15 wt% P942 77 N 46 N 28 N 18 N 14 N 0,11 % 0,56 % 2,7 % 5,5 % V15* 0.13 wt% P942 80 N 19 N 14 N < 10 N 0,44 % 1,28 % 3,9 % V16* KCI raw material (without additive) 62 N 14 N < 10 N 1,34 % 3,14 % # = test number; 1 d / 70% RH = 1 day storage at 70% relative humidity 1 d / 71% RH = 1 day storage at 71% relative humidity 1 d / 72% RH = 1 day storage at 72% relative humidity 1 d / 73% RH = 1 day storage at 73% relative humidity *each with approximately 6 wt% water; A11 = sodium carbonate anhydrous; P942 = trisodium phosphate dodecahydrate

[0077] Table 2 compares the specific effect of the combination of anhydrous sodium carbonate and trisodium phosphate dodecahydrate compared to the single trisodium phosphate dodecahydrate. The product from test 14 exhibits significantly better burst strengths—even at higher relative humidities—than the products from comparative tests V15 and V16. The moisture absorption after one day is 0.11% (70% relative humidity) and 0.56% (71% relative humidity). Table 3: Factory trials of potassium chloride granules with anhydrous sodium carbonate and anhydrous trisodium phosphate from filter-moist potassium chloride (KCl) raw material* under different weather conditions (breaking strengths in N and moisture absorption in %) Additive unweathered 1 day / 70%RH 1 day / 72%RH 1 day / 73%RH 1 day / 74%RH 17* 0.16 wt% A11 + 0.15 wt% P941 90 N 52 N 20 N 16 N 12 N 0,09 % 3,0 % 3,9 % 4,8 % 18* 0.16 wt% A11 + 0.08 wt% P941 82 N 50 N 19 N 19 N 17 N 0,10 % 3,1 % 3,9 % 5,1 % V19* 0.15 wt% P941 91 N 46 N <10 N 0,11 % 3,6 % V20* 0.30 wt% P941 88 N 44 N <10 N 0,09 % 3,5 % V21* 0.16 wt.% A11 89 N 45 N < 10 N < 10 N 0,10 % 3,1 % 4,3 % V22* KCI raw material (without additive) 62 N 14 N <10 N 1,34 % 3,14 % # = test number; 1 d / 70% RH = 1 day storage at 70% relative humidity 1 d / 72% RH = 1 day storage at 72% relative humidity 1 d / 73% RH = 1 day storage at 73% relative humidity 1 d / 74% RH = 1 day storage at 74% relative humidity *each with approximately 6 wt% water; A11 = sodium carbonate anhydrous; P941 = trisodium phosphate (anhydrous)

[0078] Table 3 compares the specific effect of the combination of sodium carbonate (anhydrous) and trisodium phosphate (anhydrous) compared to the single trisodium phosphate (anhydrous). The products from tests 17 and 18 demonstrate better fracture strength – even at higher relative humidities – than the products from comparative tests V19, V20, V21, and V22. The moisture absorption after one day is 0.09% and 0.10% (70% relative humidity) and 3.0% and 3.1% (72% relative humidity). Table 4: Laboratory tests with potassium chloride granules with the additives sodium carbonate anhydrous and trisodium phosphate (anhydrous) and micronutrients** Additive Point loads - unweathered Point loads - weathered Moisture absorption 23* 0.16 wt.% A11 + 0.32 wt.% P941 + 0.5 wt.% B 0.39 kN 0.41 kN 0,16% *with 8 wt.% water; ** For comparative tests see No. 1 and V11 A11 = sodium carbonate (anhydrous); P941 = trisodium phosphate (anhydrous), B = borax (anhydrous), calculated as boron

Claims

1. A process for the preparation of potassium chloride granulate from a crystalline potassium chloride raw material, comprising the steps of treating the potassium chloride raw material prior to granulation with at least one alkali metal carbonate and at least one phosphate additive, selected from alkali metal monophosphates, alkali metal pyrophosphates, linear alkali metal polyphosphates and mixtures thereof, in the presence of water, wherein the content of water during the treatment of the crystalline potassium chloride raw material with the alkali metal carbonate and the phosphate additive is in the range from 2 to 15% by weight, relative to the solid portion of the potassium chloride raw material, wherein the alkali metal carbonate is used in an amount of from 0.05 to 1% by weight and the phosphate additive is used in an amount of from 0.025 to 2% by weight, relative to the solid portion of the potassium chloride raw material.

2. The process according to claim 1, wherein the alkali metal carbonate is selected from anhydrous sodium carbonate, sodium carbonate monohydrate and sodium carbonate decahydrate, wherein the alkali metal carbonate is particularly anhydrous sodium carbonate.

3. The process according to claim 1 or 2, wherein the phosphate additive is selected from tri-sodium phosphate, tri-sodium phosphate-dodecahydrate, tri-sodium phosphate-hexahydrate, sodium pyrophosphate, sodium pyrophosphate-dodecahydrate and sodium-tri polyphosphate (STPP), wherein the phosphate additive is particularly tri-sodium phosphate.

4. The process according to any one of the preceding claims, wherein a quantity of 0.1% to 0.7% by weight of the alkali metal carbonate, relative to the solid portion of the potassium chloride raw material, is used.

5. The process according to any one of the preceding claims, wherein a quantity of 0.05% to 1.5% by weight of the phosphate additive, relative to the solid portion of the potassium chloride raw material, is used.

6. The process according to any one of the preceding claims, wherein the water content during the treatment of the crystalline potassium chloride raw material with the alkali metal carbonate and the phosphate additive lies in the range of 4 to 9% by weight, relative to the solid portion of the potassium chloride raw material.

7. The process according to any one of the preceding claims, wherein the alkali metal carbonate is used in the form of a powder and / or an aqueous solution.

8. The process according to any one of the preceding claims, wherein the phosphate additive is used in the form of a powder and / or an aqueous solution.

9. The process according to any one of the preceding claims, wherein the potassium chloride raw material contains 0.01 to 1.0% by weight, in particular 0.1 to 0.7% by weight, of magnesium salts and calcium salts as well as mixtures thereof, in particular present as MgCl2 and CaCl2, in each case relative to KCl and calculated as MgCl2, or CaCl2.

10. The process according to any one of the preceding claims, wherein potassium chloride raw material is used for granulation, wherein at least 90% by weight of the potassium chloride powder raw material has a particle size less than 2 mm.

11. The process according to any one of the preceding claims, wherein the granulation encompasses a press agglomeration of the potassium chloride raw material.

12. The process according to any one of the preceding claims, wherein the alkali metal carbonate is added to a moist potassium chloride raw material.

13. The process according to any one of the preceding claims, wherein the phosphate additive is added to a moist potassium chloride raw material.

14. The process according to claim 12 or 13, wherein the moist potassium chloride raw material is dried after the addition of the alkali metal carbonate and the phosphate additive and before the granulation.

15. The process according to any one of the preceding claims, wherein at least one micronutrient, in particular a micronutrient containing boron, is added to the potassium chloride raw material before or during the granulation.

16. Potassium chloride granulate, obtainable by a process in accordance with any one of claims 1 to 15.

17. Use of a combination of at least one alkali metal carbonate, at least one phosphate additive, selected from alkali metal monophosphates, alkali metal pyrophosphates, linear alkali metal polyphosphates as well as mixtures thereof, and water for reducing a moisture uptake of potassium chloride granulate.

18. Use of a combination of at least one alkali metal carbonate, at least one phosphate additive, selected from alkali metal monophosphates, alkali metal pyrophosphates, linear alkali metal polyphosphates as well as mixtures thereof, and water for increasing the breaking strength or bursting strength of potassium chloride granulate upon exposure to elevated humidity levels of 70 % RH (relative humidity) or higher.